In apparatus for use in treating sewage, one of the primary or initial treatment steps requires separating grit materials from the liquid and organic materials. These grit materials suspended in the liquid sewage are one of the most difficult materials to handle and are principally made up of sand, soil and other coarse sediment, including some hard granular organic materials which reach a sewage treatment plant. It is desired that this material be removed during the initial stages of treatment because this material cannot be treated or reduced in size and because it is hard and abrasive and therefore results in wear of pumps and any other mechanical devices used in the waste treatment process.
One form of apparatus for use in separating grit from sewage is illustrated in the Weis U.S. Pat. Nos. 4,107,038 and 3,941,698 and generally comprises a settling bin or tank. Liquid sewage is caused to flow into the tank and around the generally circular tank in a controlled pattern. As the liquid material flows through the tank, the grit material, which has a density greater than that of the other materials, tends to settle to the bottom of the tank. This grit is then pumped out of the tank or otherwise removed.
One of the problems associated with prior art grit settling basins is in relation to the means used for maintaining a proper flow of liquid through the grit settling basin such that organic solids remains suspended in the liquid while the heavier grit material will settle out.
The prior art grit basins commonly comprise a large diameter cylindrical tank positioned over and concentric to a small diameter cylindrical tank. It is intended that the sewage will flow into the large diameter cylindrical tank and the grit material will settle out and fall into the lower small diameter cylindrical tank.
One of the problems which may occur in the prior art constructions is that solid organic material has a tendency to settle into the lower smaller diameter cylindrical tank. This is undesirable in that this material tends to retard the settling of the grit material from the liquid and also tends to clog the lower cylindrical tank. On the other hand, if the velocity of the liquid flow is increased sufficiently to cause the solid organic materials to remain in suspension, the inorganic grit material may not settle out properly and may also remain in suspension in the liquid.